Mineral lubricating oils have been modified by a vast array of additives for purposes of improving viscosity index, thermal stability, oxidation stability, detergency, and other properties. The viscosity index is highly important especially in multi-grade oils in order to provide lubricating oil compositions having much flatter viscosity-temperature curves than the unmodified oils. It is especially vital that the lubricating oil compositions exhibit a specified range of viscosities at relatively low temperatures. A "multi-grade lubricant" designates those lubricants which meet a 0.degree. F viscosity specification and a 210.degree. F viscosity specification, such as is shown for motor oils by the following table derived from SAE, J300a taken from the SAE Handbook for 1969:
______________________________________ SAE SAE Oil Grade Viscosity at Oil Grade Viscosity at Specification 0.degree. F, poises Specification 210.degree. F, SUS ______________________________________ -- -- 20 45-58 5W 12 maximum 30 58-70 10W 12-24 40 70-85 20W 24-96 50 85-110 ______________________________________
According to the table, for example, an SAE 10W/50 oil must have a viscosity at 0.degree. F between 12 and 24 poises and a viscosity at 210.degree. F of between 85 and 110 SUS.
One particularly useful viscosity-index improver is a selectively hydrogenated styrene-isoprene two block copolymer, such as those disclosed in U.S. Pat. Nos. 3,763,044, and 3,772,196. Lubricants which contain these VI improvers have excellent viscosity indices, as derived from their 100.degree. F and 210.degree. F kinematic viscosities. When the viscosity is plotted against temperature, using charts described in ASTM Standard D341, the relationship is essentially linear from 100.degree. F to 210.degree. F. However, for certain of these selectively hydrogenated styrene-isoprene copolymers, at temperatures approaching 300.degree. F, the lubricant viscosity deviates from linearity and begins to drop. The temperature where this occurs is referred to as the break point temperature (T.sub.b). A modified copolymer has now been prepared that gives a more linear temperature-viscosity relationship at temperatures above 210.degree. F and thereby a greater 300.degree. F kinematic viscosity.